1. Field of the Invention
The invention relates to a copying machine and a facsimile machine, various kinds of image forming apparatuses such as a printer and a developing device applied to the image forming apparatuses. More particularly, it relates to a developing device and an image forming apparatus including a mechanism for resupplying recycled toner particles.
2. Description of the Related Art
As a developing device applied to image forming apparatuses such as a copying machine and a facsimile machine, a developing device disclosed in a Japanese Unexamined Patent Publication No. 2001-235933 is known. The developing device includes a housing for supplying toner particles to a photoconductive drum of an image forming apparatus, and a toner cartridge detachably mounted on the housing for supplying toner particles. In a bottom portion of the toner cartridge, an openable and closable toner discharging section is formed. In the housing, a toner inlet section corresponding to the toner discharging section is formed. When the toner discharging section is opened by mounting the toner cartridge on the housing, toner particles stored in the toner cartridge is supplied to a predetermined circulating conveyance passage provided in the housing.
The circulating conveyance passage is formed to be a groove extending parallel in an axial direction of the photoconductive drum and having an open end in its upper portion. The circulating conveyance passage has a front conveyance passage extending along a developing roller, and a rear conveyance passage extending parallel to the front conveyance passage and passing a position to receive developer particles from the developer inlet section. In each of the conveyance passages, a spiral feeder having a spiral fin spirally formed on a periphery of the feeder shaft is provided. Toner particles (toner particles and carrier particles in the case of two-component developer particles) are circulated in the front conveyance passage and the rear conveyance passage in accordance with a rotation of the spiral fin about the feeder shaft.
Toner particles received by the rear conveyance passage from the toner cartridge through the toner inlet section are moved to the front conveyance passage by a rotation of the spiral feeder about the feeder shaft and then sent to the front conveyance passage through a communicating passage at a downstream of the rear conveyance passage. Toner particles moved to the front conveyance passage are supplied to a peripheral surface of the developing roller while being conveyed in accordance with a driving of the spiral feeder in a direction opposite to the conveyance direction of the rear conveyance passage. Remainder of toner particles are conveyed back to the rear conveyance passage at a downstream end of the front conveyance passage. In the case of using two-component developer particles, toner particles and carrier particles are stirred and mixed in the rear conveyance passage. Then, a required amount of developer particles (toner particles and carrier particles) are supplied to the peripheral surface of the developing roller in the front conveyance passage. Remainder of developer particles are conveyed back to the rear conveyance passage.
In a developing device of the Japanese Unexamined Patent Publication No. 2001-235933 having such fundamental construction, a conveyance power reduction portion which is so constructed as to reduce a conveyance power locally is provided at a location which is on downstream of the toner inlet section in a conveyance direction of the spiral feeder provided in the rear conveyance passage. The conveyance power reduction portion is provided for causing retaining of toner particles on upstream thereof. Since the retaining of toner particles occurs between the conveyance power reduction portion and the toner inlet section, retained toner particles block the toner inlet section to thereby restrict supply of toner particles when enough amount of toner particles exist in the developing device. On the contrary, when toner particles in the developing device are consumed and amount of toner particles is reduced, the retained toner particles are also reduced. Accordingly, a space is formed between the portion where toner particles are retained and the toner inlet section so that toner particles fall down from the toner cartridge and supplied into the developing device. When enough amount of toner particles are supplied into the developing device, retaining of toner particles occurs and the toner inlet section is closed so that the supply of toner particles is restricted. As can be seen, the amount of toner particles supplied to the developing device from the toner cartridge is automatically adjusted in accordance with amount of toner particles existing in the developing device.
Meanwhile, there is a known mechanism aimed for reusing toner particles by recovering toner particles resided on a photoconductive drum after a transferring process and resupplying the same to the developing device as recycled toner particles (for example, refer to Japanese Unexamined Patent Publication Nos. 2001-235933, 2000-29312, HEI8-54809, HEI9-197786 and HEI5-249828). Adopting such mechanism provides an advantage that toner particles resided after the transferring process can be utilized effectively.
However, since the conveyance power reduction portion described above is provided so as to retain toner particles and restrict supply of toner particles from the toner cartridge in the developing device described in Japanese Unexamined Patent Publication No. 2001-235933, a problem of clogging (a phenomenon where toner particles are rammed down and clog a toner inlet section so that toner particles are not supplied from the toner cartridge even though toner particles run short) has been arose when the developing device is, for example, in a high-temperature environment or in the condition where a low-darkness printing is executed for a long time so that few toner particles are consumed.
Especially in the case where toner particles resided on the photoconductive drum after a transferring process is recovered and supplied again to the developing device as recycled toner particles, there has been a problem that clogging is likely to occur since a fluidity of recycled toner particles is generally lowered. Further, since recycled toner particles have low charging characteristic, there has been a problem that toner particles are not well mixed (well attached with charges) with carrier particles. This is because recycled toner particles undergo various kinds of stresses before they are resupplied to the developing device and various changes such as separation and burial of agent particles and aggregation of toner particles so that fluidity and charging characteristic are changed from the initial state.